Macromolecules, man-made or modified by man, are an important part of the human environment; for these reasons the biological effects of representative water soluble macromolecules have been studied. The results show that foreign, non-degradable macromolecules are partly taken up by human cells grown in glass and stay associated with them for many generations. In animals foreign non-degradable macromolecules are rapidly distributed in the body and only a small portion of the injected macromolecules can be excreted; in organs the foreign macromolecules are found even a month after application. This situation must be compared with the finding that potential beneficiary effects on animals of a non-degradable polymer (poly-9-vinyladenine protection against viral leukemia) are of only about one day's duration. On the other hand, only negligible beneficiary effects in protection against viruses were obtained with macromolecules which are degraded in less than an hour. Thus the search for potential polymeric drugs should concentrate on compounds of intermediate stability.